Animal treatment with various antioxidants is known to inhibit tumorigenesis induced by benzo (a) pyrene (BP) and other carcinogens. We studied effects of butylated hydroxyanisole (BHA on in vivo binding of BP to DNA and protein. BHA treatment of mice given BP resulted in decreases in total DNA binding by 21 and 32 percent, respectively, in lung and liver. The extent of reduction in total DNA-binding did not appear to correlate with inhibition of tumor formation. Analysis of the effect of BHA treatment on the nature of BP-DNA adducts showed that the adducts were affected to different extents. Amounts of (plus)-BP-7 beta, 8 alpha-diol-9 alpha, 10 alpha-epoxide-DNA (BPDE-I) adduct formed were decreased 56 percent in lung, while amounts of adducts due to recycled BP phenols, quinones and BP 4,5-oxide were unaltered. Thus, specific reduction in BPDE-I adduct appears to correlate with a 53 percent inhibition of pulmonary adenoma formation. Antioxidants appear to inhibit tumor formation in lung by specifically inhibiting amount of BPDE-I adduct formed. In rats predominant BP-DNA adduct(s) formed in lung and liver resulted from interaction of a recycled-BP phenol with DNA. Only small amounts of BPDE-I adduct were formed. Treatment of rats with BHA increased total DNA binding and amounts of all DNA-adducts formed.